This invention relates to nuclear gauging; and more particularly to a method of apparatus that provides height compensation in nuclear gauging applications wherein the average atomic number of a scattering medium is being monitored, such as nuclear mine detection.
In nuclear gauging, the number and energy distribution of gamma and/or x-ray photons detected at a given point is space per unit time from a radiation source is a function of source type, source-detector geometry, and the density and elemental composition of the surrounding media. By keeping any three of these variables constant, it is possible to design a nuclear gauge which will monitor the fourth. Such nuclear gauges are routinely used in industry to monitor, for example, the thickness of sheet metal or the void fraction of fluid flow through a pipe. Such nuclear gauges have proven highly successful and are routinely utilized; however, attempts to apply nuclear gauging techniques to, for example, the detection of nonmetallic landmines have not been as successful, due to certain problems encountered when attempting to apply prior-art nuclear gauging techniques to such an application.
Nonmetallic landmines differ from the surrounding soil medium in elemental composition. Explosives are composed entirely of light elements such as hydrogen, carbon, nitrogen, and oxygen (H, C, N, O), while soils consist of oxygen and somewhat heavier elements, such as aluminum, silicon, and iron (O, Al, Si, Fe). The average atomic number (Z) of explosives is approximately 5, whereas that of typical soils is approximately 11. Since the macroscopic photoelectric cross section varies approximately as the fourth power of atomic number, the photon flux backscattered from a half space above which a source of gamma radiation has been suspended will vary appreciably with the inverse of the atomic number of that half space in the energy region where the photoelectric cross section is an appreciable fraction of the total cross section. Considering the four variables mentioned above and the fact that three of the four variables are to be held constant to monitor the fourth, it is obvious that a practical mine detector can be designed if the source type, source-detector geometry, and the density of the soil can be held constant or variations compensated for while the elemental differences between the soil and landmines are monitored.
In designing a gamma backscatter mine detector, both the gamma source and the geometry of the sensor are determined. Unfortunately, the source-detector geometry depends, not only on the sensor geometry, but also on the location of the sensor relative to the soil interface. Since in operation the height of the sensor above the soil cannot from a practical standpoint be held constant, some means had to be devised to compensate for height variations. (Soil density variations do occur, but their effect is small compared to the effect of Z variations. On the other hand, height changes of as little as an inch can, given certain sensor geometries, entirely mask a change in Z by a factor of 2.)
This invention provides a method for compensating for the effects of height variations in nuclear landmine detectors and delineates one type of apparatus suitable for implementing this method. While the invention is ideally suited for nuclear landmine detection, the invention is also useful in more conventional nuclear gauging applications where the distance from a backscatter sensor to the material of interest cannot be held constant.